CVS, which is mostly used for managing program source versions, offers the possibility to keep copies of the files on multiple computers. Accordingly, it is also suitable for data synchronization. CVS maintains a central repository on the server in which the files and changes to files are saved. Changes that are performed locally are committed to the repository and can be retrieved from other computers by means of an update. Both procedures must be initiated by the user.

CVS is very resilient to errors when changes occur on several computers. The changes are merged and, if changes took place in the same lines, a conflict is reported. When a conflict occurs, the database remains in a consistent state. The conflict is only visible for resolution on the client host.

When no version control is needed but large directory structures need to be synchronized over slow network connections, the tool rsync offers well-developed mechanisms for transmitting only changes within files. This not only concerns text files, but also binary files. To detect the differences between files, rsync subdivides the files into blocks and computes checksums over them.

The effort put into the detection of the changes comes at a price. The systems to synchronize should be scaled generously for the usage of rsync. RAM is especially important.

Two different models are commonly used for distributing data. In the first model, all clients synchronize their files with a central server. The server must be accessible by all clients at least occasionally. This model is used by CVS.

The other possibility is to let all networked hosts synchronize their data between each other as peers. rsync actually works in client mode, but any client can also act as a server.

CVS and rsync are also available for many other operating systems, including various Unix and Windows systems.

In CVS, the data synchronization is started manually by the user. This allows fine control over the data to synchronize and easy conflict handling. However, if the synchronization intervals are too long, conflicts are more likely to occur.

Conflicts only rarely occur in CVS, even when several people work on one large program project. This is because the documents are merged on the basis of individual lines. When a conflict occurs, only one client is affected. Usually conflicts in CVS can easily be resolved.

There is no conflict handling in rsync. The user is responsible for not accidentally overwriting files and manually resolving all possible conflicts. To be on safe side, a versioning system like RCS can be additionally employed.

In CVS, new directories and files must be added explicitly using the command cvs add. This results in greater user control over the files to synchronize. On the other hand, new files are often overlooked, especially when the question marks in the output of cvs update are ignored due to the large number of files.

An additional feature of CVS is that old file versions can be reconstructed. A brief editing remark can be inserted for each change and the development of the files can easily be traced later based on the content and the remarks. This is a valuable aid for theses and program texts.

A sufficient amount of free space for all distributed data is required on the hard disks of all involved hosts. CVS require additional space for the repository database on the server. The file history is also stored on the server, requiring even more space. When files in text format are changed, only the modified lines need to be saved. Binary files require additional space amounting to the size of the file every time the file is changed.

Experienced users normally run CVS from the command line. However, graphical user interfaces are available for Linux, such as cervisia, and for other operating systems, like wincvs. Many development tools, such as kdevelop, and text editors, such as Emacs, provide support for CVS. The resolution of conflicts is often much easier to perform with these front-ends.

rsync is rather easy to use and is also suitable for newcomers. CVS is somewhat more difficult to operate. Users should understand the interaction between the repository and local data. Changes to the data should first be merged locally with the repository. This is done with the command cvs update. Then the data must be sent back to the repository with the command cvs commit. Once this procedure has been understood, newcomers are also able to use CVS with ease.

During transmission, the data should ideally be protected against interception and manipulation. CVS and rsync can easily be used via ssh (secure shell), providing security against attacks of this kind. Running CVS via rsh (remote shell) should be avoided. Accessing CVS with the pserver mechanism in insecure networks is likewise not advisable.

CVS has been used by developers for a long time to manage program projects and is extremely stable. Because the development history is saved, CVS even provides protection against certain user errors, such as unintentional deletion of a file.

The server is the host on which all valid files are located, including the latest versions of all files. Any stationary workstation can be used as a server. If possible, the data of the CVS repository should be included in regular backups.

When configuring a CVS server, it might be a good idea to grant users access to the server via SSH. If the user is known to the server as tux and the CVS software is installed on the server as well as on the client, the following environment variables must be set on the client side:

CVS_RSH=ssh CVSROOT=tux@server:/serverdir

The command cvs init can be used to initialize the CVS server from the client side. This needs to be done only once.

Finally, the synchronization must be assigned a name. Select or create a directory on the client exclusively to contain files to manage with CVS (the directory can also be empty). The name of the directory is also the name of the synchronization. In this example, the directory is called synchome. Change to this directory and enter the following command to set the synchronization name to synchome:

cvs import synchome tux wilber

Many CVS commands require a comment. For this purpose, CVS starts an editor (the editor defined in the environment variable $EDITOR or vi if no editor was defined). The editor call can be circumvented by entering the comment in advance on the command line, such as in the following example:

cvs import -m 'this is a test' synchome tux wilber

The synchronization repository can now be checked out from all hosts with cvs co synchome. This creates a new subdirectory synchome on the client. To commit your changes to the server, change to the directory synchome (or one of its subdirectories) and enter cvs commit.

By default, all files (including subdirectories) are committed to the server. To commit only individual files or directories, specify them as in cvs commit file1 directory1. New files and directories must be added to the repository with a command like cvs add file1 directory1 before they are committed to the server. Subsequently, commit the newly added files and directories with cvs commit file1 directory1.

If you change to another workstation, check out the synchronization repository if this has not been done during an earlier session at the same workstation.

Start the synchronization with the server with cvs update. Update individual files or directories as in cvs update file1 directory1. To see the difference between the current files and the versions stored on the server, use the command cvs diff or cvs diff file1 directory1. Use cvs -nq update to see which files would be affected by an update.

Here are some of the status symbols displayed during an update:

The status M indicates a locally modified file. Either commit the local copy to the server or remove the local file and run the update again. In this case, the missing file is retrieved from the server. If you commit a locally modified file and the file was changed in the same line and committed, you might get a conflict, indicated with C.

In this case, look at the conflict marks (»> and «<) in the file and decide between the two versions. As this can be a rather unpleasant job, you might decide to abandon your changes, delete the local file, and enter cvs up to retrieve the current version from the server.

This section merely offers a brief introduction to the many possibilities of CVS. Extensive documentation is available at the following URLs:

rsync can be operated in two different modes. It can be used to archive or copy data. To accomplish this, only a remote shell, like ssh, is required on the target system. However, rsync can also be used as a daemon to provide directories to the network.

The basic mode of operation of rsync does not require any special configuration. rsync directly allows mirroring complete directories onto another system. As an example, the following command creates a backup of the home directory of tux on a backup server named sun:

 rsync -baz -e ssh /home/tux/ tux@sun:backup
 

The following command is used to play the directory back:

 rsync -az -e ssh tux@sun:backup /home/tux/
 

Up to this point, the handling does not differ much from that of a regular copying tool, like scp.

rsync should be operated in “rsync” mode to make all its features fully available. This is done by starting the rsyncd daemon on one of the systems. Configure it in the file /etc/rsyncd.conf. For example, to make the directory /srv/ftp available with rsync, use the following configuration:

gid = nobody
uid = nobody
read only = true
use chroot = no
transfer logging = true
log format = %h %o %f %l %b
log file = /var/log/rsyncd.log

[FTP]
        path = /srv/ftp
        comment = An Example
 

Then start rsyncd with rcrsyncd start. rsyncd can also be started automatically during the boot process. Set this up by activating this service in the runlevel editor provided by YaST or by manually entering the command insserv rsyncd. rsyncd can alternatively be started by xinetd. This is, however, only recommended for servers that rarely use rsyncd.

The example also creates a log file listing all connections. This file is stored in /var/log/rsyncd.log.

It is then possible to test the transfer from a client system. Do this with the following command:

rsync -avz sun::FTP
 

This command lists all files present in the directory /srv/ftp of the server. This request is also logged in the log file /var/log/rsyncd.log. To start an actual transfer, provide a target directory. Use . for the current directory. For example:

rsync -avz sun::FTP .
 

By default, no files are deleted while synchronizing with rsync. If this should be forced, the additional option --delete must be stated. To ensure that no newer files are deleted, the option --update can be used instead. Any conflicts that arise must be resolved manually.

Important information about rsync is provided in the man pages man rsync and man rsyncd.conf. A technical reference about the operating principles of rsync is featured in /usr/share/doc/packages/rsync/tech_report.ps. Find the latest news about rsync on the project Web site at http://rsync.samba.org/.

If you want Subversion or other tools, download the the SDK. Find it at http://developer.novell.com/wiki/index.php/SUSE_LINUX_SDK.